1. Field of the Invention
The present invention relates to a field-emission type cold cathode, and in particular, to one having a gate electrode in the vicinity of the emitter which emits electrons. The present invention also relates to applications of such a field-emission type cold cathode.
This application is based on Patent Application No. Hei 10-266950 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
The field-emission type cold cathode is a device comprising a sharp conical emitter and a gate electrode which is formed in the vicinity of the emitter and has a sub-micron-order aperture. In such a device, a high field is concentrated at the pointed end of the emitter, from which electrons are emitted in a vacuum. The emitted electrons are received by an anode electrode which is separately formed. Recently, such devices have become smaller based on the development of the fine manufacturing techniques, and such smaller devices are widely used as constituents of subminiature triode electron tubes or electron guns used in thin display devices.
In conventional field-emission type cold cathodes having a gate diameter of approximately 1 xcexcm, a voltage of approximately 100 V is applied between the emitter and the gate electrode so as to emit electrons from the head point of the emitter. However, with an operating voltage of 100 V or more, the operating conditions are limited with respect to the power consumption, control circuits, or the like; thus, operation with a lower voltage has been required. An example method for satisfying such a requirement is to provide gate electrode with a fine aperture (i.e., with a very small diameter). However, making a fine aperture is accompanied with having a thinner thickness of the insulating film between the emitter and gate electrode, thereby degrading the withstand voltage. Therefore, a method for making a gate electrode of a fine diameter without degrading the withstand voltage has been required.
In addition, the emitter and the gate electrode are separately but closely arranged, so that a discharge may occur between the emitter and the gate electrode. If such a discharge produces an excess current flowing through the emitter or the gate electrode, the material of the electrode may melt and short-circuit breakdown may occur between the emitter and gate electrode.
In order to prevent such a failure, that is, to suppress an excess current due to the discharge, it is effective to provide another element for suppressing current, at the emitter or gate electrode. A typical known method is to form a resistor connected to the emitter. However, in this method, an area for forming a resistor is necessary, and the current-suppressing effect is also effective during the normal operation so that the operating voltage may rise. In these circumstances, a vertically-formed current control element having non-linear current/voltage characteristics has peen proposed.
Japanese Unexamined Application, First Publication, No. Hei 10-12128 discloses an example of a field-emission type cold cathode comprising such a vertically-formed current control element.
FIGS. 8A to 8F are sectional views for explaining the manufacturing processes in the first conventional example performed in turn.
As shown in FIG. 8A, masking film 14 consisting of an oxide film is formed on silicon substrate 1 at a thickness of 1 xcexcm, and then the patterning of masking film 14 is performed using a resist or the like, so that the substrate 1 is exposed. The anisotropic etching using the masking film 14 as a mask (for the etching) is performed on the exposed substrate so that trench 4 of 10 xcexcm depth is formed.
Next, a BPSG (boron-phosphorus silicate glass) film of 2 xcexcm thickness is formed using the LPCVD (low pressure chemical vapor deposition) method, and the etchback process is performed until the BPSG film 5 is embedded within the trench 4, as shown in FIG. 8B.
Next, as shown in FIG. 8C, oxide film 6 is deposited at a thickness of approximately 400 nm by using the CVD method, and gate electrode film 7 is further deposited at a thickness of 200 nm by using a spattering method, so as to perform the patterning of the electrode having a desired shape.
Then, as shown in FIG. 8D, gate aperture 8 having an approximately 0.5 xcexcm diameter is formed in an area where an emitter is provided later, by selectively etching the gate electrode 7 and oxide film 6, and a sacrificial layer 9 such as an alumina film is deposited on the top face of the gate electrode 7 and on the side walls of the gate electrode 7 and oxide film 6 by performing the rotational vapour deposition from a slantwise direction. An emitter material such as molybdenum is then deposited from a vertical direction by the vapour deposition, so that emitter 10a and extra emitter material 10b are respectively formed on the substrate and the sacrificial layer.
Lastly, as shown in FIG. 8F, the sacrificial layer is etched using phosphoric acid or the like, so that the emitter material 10b is lifted off and a field-emission type cold cathode is obtained.
In the above conventional example, the portion surrounded by trench 4 functions as a discharge-current suppressing element having non-linear current/voltage characteristics, thereby preventing a short-circuit breakdown of the device.
FIG. 9 shows an example of a structure for further improving the operational characteristics. In this structure, a set of oxide film 6 and gate electrode 7, having an aperture, is deposited on silicon substrate 1. A sharp conical emitter 10a is formed in the gate aperture 8, and the area of emitter 10a is surrounded by trench 4 (in substrate 1) in which BPSG film 5 is embedded. In addition, an n type area, more specifically, n type diffusion layer 12 (to which n type impurity is doped with a higher concentration than that of substrate 1) is provided in the top face of the substrate 1.
That is, in this example, the n type diffusion layer 12 is added to the structure shown in FIG. 8F. The structure shown in FIG. 9 can reduce the contact resistance at the lower part of the emitter and prevent the current (path) from concentrating at the lower part of the emitter. That is, without the n type diffusion layer 12, current concentrates at a local area when a discharge occurs, so that a voltage is applied to a local area due to the contact resistance between the emitter and the substrate, which causes a breakdown and reduces the effective length of the discharge-current suppressing element (provided in the area surrounded by the trench and having non-linear current/voltage characteristics). That is, the structure shown in FIG. 9 prevents a high field from applying to both sides of the element having a shorter effective length; thus, degradation of the withstand voltage can be prevented.
The first problem related to the conventional technique is that a thermal (i.e., thermally oxidized) oxide film having better insulating capability cannot be used for forming the insulating film below the gate electrode, so that the finer the gate diameter, the thinner the insulating film is, thereby reducing the withstanding voltage between the emitter and gate electrode. This is because the height of the emitter is in proportion to the diameter of the gate aperture. For example, if the diameter of the gate aperture decreases from 0.8 xcexcm to 0.4 xcexcm, then the thickness of the oxide film also decreases from 0.4 xcexcm to 0.2 xcexcm.
The second problem related to the conventional technique is that when the oxide film between the emitter and gate electrode (being accompanied with a finer gate diameter) becomes thinner, the creeping distance along the side wall of the oxide film becomes shorter. Generally, a surface of the oxide film, exposed in a vacuum, between the emitter and gate electrode may include a path relating to the generation of surface states, accretion, discharge on the relevant surface, or the like, that is, such a path may generate a leak current between the emitter and gate electrode. Accordingly, if the oxide film between the emitter and gate electrode is made thinner according to a finer structure and the creeping distance is made shorter, then a leak current flows between the emitter and gate electrode.
The third problem related to the conventional technique is that the withstand voltage of the discharge-current suppressing element, formed via the trench, is reduced along with the employment of a thinner oxide film between the emitter and gate electrode according to a finer structure of the device. This is because the withstand voltage with respect to the cross direction is reduced or degraded. Here, the withstand voltage in the longitudinal direction depends on the depth of the trench, while the withstand voltage in the cross direction depends on the width of the trench. These withstand voltages determine the withstand voltage of the field-emission type cold cathode.
Practically, the withstand voltage in the cross direction also depends on the width of the trench and the potential difference generated between the emitter and the trench. When the gate electrode is positively voltage-applied and the oxide film below this gate electrode is thinner, an n type channel is formed in the substrate below the oxide film and in the substrate; thus, the withstand voltage only depends on the width of the trench in this case. This dependence condition may be remarkable in a structure having an n type diffusion layer 12 on the substrate as shown in FIG. 9 so as to reduce the contact resistance between the emitter and the substrate.
Therefore, an objective of the present invention is to solve the above problems such as degradation of the withstand voltage and increase of the leak current between the emitter and gate electrode, and degradation of the withstand voltage when a discharge occurs, these phenomena usually accompanying a finer structure. A further objective of the present invention is to reduce the distance between the gate electrode and emitter, apply a higher electric field to the head point of the emitter, decrease the operational voltage, and simultaneously improve the reliability.
Therefore, the present invention provides a field-emission type cold cathode comprising:
a substrate, on a surface of which an emitter for emitting electrons is formed, for functioning as a leading (or lead) emitter electrode;
a gate electrode, formed via an insulating film on the substrate, having an aperture which surrounds the emitter via a space, and
wherein the height of a boundary, facing the space, between the insulating film and the substrate is lower than the height of the surface of the substrate on which the emitter is formed, and the cold cathode further comprising:
an insulated trench surrounding an area on which the emitter is formed, where the boundary between the insulating film and the substrate is placed between the emitter and the trench, and a part of the insulating film is present between the boundary and the trench.
Accordingly, the field-emission type cold cathode of the present invention can operate with a low operating voltage and has a high withstand voltage, by which excess current generated when a discharge occurs can be suppressed while maintaining a suitable voltage during the normal operation.
This is because sufficient space exists between the emitter and gate electrode, and the thickness of the insulating film for supporting the gate electrode can be greater than the distance between the emitter and gate electrode. Therefore, even if the device has a finer structure as a result of reducing the distance between the emitter and gate electrode, it is possible to prevent degradation of the withstand characteristics due to a decrease of the thickness of the insulating film. Additionally, the operating voltage can be lowered due to the finer structure.
In addition, a part of the insulating film (supporting the gate electrode) may be made of a material which has an etching speed different from that of the remaining portion. In this case, when a portion of the insulating film, which is close to the emitter in the gate aperture, is side-etched, the material which has a lower etching speed remains and thus has the same length as the gate electrode. Accordingly, the effective creeping distance of the insulating film in the vicinity of the emitter can be longer, thereby improving the withstand voltage with respect to the creeping surface between the emitter and gate electrode, and suppressing the generation of leak currents.
Furthermore, the insulated trench (typically, an insulating film is embedded in it) is formed in an area of substrate outside of the space around the emitter; thus, the area surrounded by the trench can function as a pinch resistor. Accordingly, the operating voltage does not increase during the normal operation, and excess current can be suppressed when a discharge occurs between the emitter and gate electrode, thereby preventing the melting destruction of the emitter or gate electrode. In addition, a part of the insulating film is present between the emitter and trench. Therefore, even if an n type diffusion layer for reducing the contact resistance is provided immediately below the emitter, the trench and the n type diffusion layer do not contact each other, and the effective width of the insulating film (in the cross direction) related to the upper portion of the trench can be greater, thereby easily suppressing the current when a discharge occurs.
In the above structure, a step of height difference may exist between the boundary and the surface on which the emitter is formed, and the portion of the step may be positioned between the insulating film and the emitter.
In addition, the insulating film supports the gate electrode, and the thickness of the insulating film is preferably greater than the distance between the emitter and the gate electrode.
Typically, the insulating film is formed on at least one of the surface of the substrate, which faces the space in which the emitter is present, and the surfaces of the gate electrode which also face the space.
It is possible that the surface of the substrate which contacts the emitter and is surrounded by the boundary has an n type area whose concentration is higher than that of the substrate.
As explained above, according to the present invention, the thickness of the insulating film which supports the gate electrode can be greater than the distance between the emitter and the gate electrode measured in the vicinity of the emitter. Therefore, the insulating capability between the emitter and the gate electrode can be improved, and the withstand voltage of a discharge-current suppressing element surrounded by the trench can be increased. Additionally, the field-emission type cold cathode with a lower operating voltage can be obtained by reducing the distance between the emitter and gate electrode.
The above field-emission type cold cathode having a high withstand voltage (with which the current can be reliably controlled when a discharge occurs), which can operate with a lower voltage, can be applied to a display device for displaying images, such as a flat panel display or a cathode ray tube. Accordingly, it is possible to provide a highly reliable display device which can operate with a lower voltage with stable current characteristics.
The above field-emission type cold cathode having a high withstand voltage (with which the current can be reliably controlled when a discharge occurs), which can operate with a lower voltage, can also be applied to a travelling-wave tube having an amplifying function by using an electron gun. Accordingly, it is possible to provide a highly reliable travelling-wave tube which can operate with a lower voltage with stable current characteristics.